1. Field of the Invention
The present invention relates generally to a channel coding method and apparatus. More particularly, the present invention relates to a method and apparatus for preventing a performance loss of a Reed-Solomon (R-S) code caused when a time division multiplexing point serviced between a transmitter and a receiver is changed in a Code Division Multiple Access (CDMA) mobile communication system using R-S codes.
2. Description of the Related Art
A mobile communication system has evolved from an early system that provides voice service into an advanced system capable of providing data service. The mobile communication system has now developed into a system that can provide broadcast service together with various data services. The system providing broadcast service is under standardization in a 3rd Generation Partnership Project 2 (3GPP2) group. Broadcast service proposed in the 3GPP2 group is called “Broadcast and Multicast (BCMC) Service” in a CDMA2000 1x Rev. D standard. In addition to the CDMA2000 1x Rev. D standard, a High Rate Broadcast-Multicast Packet Data (HRPD) Rev. A standard for the synchronous CDMA system also provides BCMC Service.
In the following description, BCMC service proposed in both the CDMA2000 1x Rev. D standard and the HRPD Rev. A standard for the CDMA system will be referred to as “broadcast service.”
The broadcast service is provided in which broadcast data based on a frame having a period of, for example, 20 ms is transmitted by Time Division Multiplexing (TDM). For channel coding, the broadcast service can use an R-S code, which is an error correction code, well known as an outer code, aside from an inner code such as a convolutional code or a turbo code. Accordingly, if the TDM scheme is used in transmitting broadcast data, a receiver selectively receives a minimum number of frames, contributing to improvement in the receiver's reception performance. However, in order to prevent possible consecutive transmission errors of broadcast data, the broadcast service uses the R-S code as currently proposed in the CDMA2000 1x Rev. D standard. An exemplary implementation of the present invention relates to a channel coding method. The channel coding method improves positions of information data sequentially input to an encoder when some of channel coded bits are defective, thereby preventing a loss of the information data and improving the performance. In particular, the present invention relates to a physical channel format used for transmission based on a cdma2000 High Rate Broadcast-Multicast Packet Data (HRPD) standard for the synchronous CDMA mobile communication system, and efficiently encodes the R-S code used for Broadcast/Multicast Service (BCMCS), thereby to prevent a performance loss of the R-S code caused when a time division multiplexing point serviced between a transmitter and a receiver is changed. This technology is not limited only to the CDMA2000 HRPD standard for the synchronous CDMA mobile communication system, but can be generally applied when a portion of channel coded data cannot be transmitted. A description will be made based on the CDMA2000 High Rate Broadcast-Multicast Packet Data (HRPD) standard (in this specification, it will be referred to as “BCMC” unless stated otherwise).
A general BCMC transmitter has an R-S error control block buffer for each individual logical channel. For a plurality of logical channel bits input to the R-S error control block buffer, R-S coded symbols for each individual channel are output to a TDM multiplexer. The TDM multiplexer TDM-multiplexes the input symbols for each individual logical channel. The channel symbols generated by the TDM multiplexer undergo additional transmission coding and modulation processes through an inner encoder before being transmitted.
A general BCMC receiver includes a process of demodulating the data transmitted by the TDM multiplexer of the BCMC transmitter, and an R-S decoding process of performing inner decoding on the demodulated data and then inputting a desired logical channel to its R-S error control block buffer using a logical channel selector. With reference to FIG. 1, a description will now be made of a transmission method based on the BCMC standard.
FIG. 1 is a diagram illustrating an exemplary TDM transmission method according to the BCMC standard. The TDM transmission method is classified according to interlace, which each interlace is allocated Multiplex and BurstLength. The term “interlace” refers to a slot of a physical channel that is transmitted in units of time slots in a 1x EV-DO network, and the term “multiplex” refers to an index obtained by logically dividing the interlace. In addition, the term “BurstLength” refers to the number of repeated transmissions for a logical broadcast channel created with the interlace and the multiplex. Each interlace is transmitted in units of slots, and numerals under the interlace represent corresponding slot indexes. In addition, alphabets A, B, C, . . . shown in FIG. 1 represent logical channels. In FIG. 1, an interlace-0 100 includes 4 multiplexed channels A, B, C and D; an interlace-1 102 includes 4 multiplexed channels E, F, G and H; an interlace-2 104 includes 4 multiplexed channels A, K, L and M; and an interlace-3 106 includes 4 multiplexed channels O, P, Q and R. A length of each multiplexed channel included in the interlace-0 100 is set to BurstLength0[i] (where i={0, 1, 2, 3}). TotalBurstLength is a sum of all BurstLengths in each individual interlace.
FIG. 2 is a diagram illustrating a structure of an R-S error control block buffer for one logical channel when an R-S code, which is an outer code, is used in BCMC.
In FIG. 2, N (200) denotes a length of an R-S code, K (202) denotes a length of an information symbol, and R (204) denotes a length of a parity symbol. In addition, M (206) denotes a value corresponding to a size of a horizontal axis of the block, where M={1, 2, . . . , 16}. In FIG. 2, the actual data before R-S coding has a size of K×M, and a parity portion created after the R-S coding has a size of R×M.
Broadcast security packets 208 are transmitted to a receiver over one logical channel in order of arrival at the R-S error control block buffer (not shown), and the broadcast security packets 208 positions in the logical channel are shown in FIG. 1. A relationship between positions of the broadcast security packets in the R-S error control block buffer and their actual transmission time points in one logical channel is defined as follows.
1) Assuming that an absolute time 0 (for example, 1980.01.01) is defined, the current structure of a TDM logical channel has been continuously maintained from the absolute time 0 to the present time.
2) For a logical channel desired to be sent, a first broadcast security packet in the R-S error control block buffer starts transmitting at a slot first allocated after the absolute time 0.
3) Assuming that the transmission started by the foregoing operation has continued up to the present time, the data in a position of a broadcast security packet in the R-S error control block buffer, which must be sent at the present time, is transmitted in a corresponding slot interval.
Several parameters for the BCMC transmission are delivered by a Broadcast Overhead Message (BOM). A description of the BOM will be made below with reference to FIG. 3.
In the BCMC service, a base station previously provides position information of broadcast data to a receiver through a BOM. The base station should necessarily transmit a control channel to the receiver every 256 slots, and transmit a BOM every 7 slots. The BOM includes BurstLength and TotalBurstLength.
FIG. 3 is a diagram illustrating a transmission time of a BOM which is additional information used for providing BCMC. The BOM, as shown in FIG. 3, is delivered basically at intervals of 7 BroadcastOverheadPeriods×256 slots=3 seconds. The BCMC can perform allocation, deallocation and position-shifting on each logical channel by modifying the BOM.
In the BCMC standard, the BOM is delivered as shown in FIG. 3. The BOM is repeatedly transmitted with the same value as long as TDM information is maintained. However, if the TDM information is changed, the BOM is transmitted with a value different from the value transmitted in the previous interval. Accordingly, when the TDM information is changed by the BOM, an R-S decoder cannot perform R-S decoding.
FIG. 4 is a diagram illustrating an R-S error control block transmission problem caused by a change in BOM format according to conventional art.
Transmission points of the data in an R-S error control block buffer are determined on the assumption that first transmission always starts after the absolute time 0. Referring to FIG. 4, based on the format defined in a received BOM, a receiver determines which position of the R-S error control block buffer the received data is located. Therefore, if the BOM is changed from BOMA 400 to BOMB 402, a position of the data last transmitted according to the format defined in the BOMA 400, which is a BOM before the change, in the R-S error control block buffer, and a position of the data first transmitted according to the format defined in the BOMB 402, which is a BOM after the change, in the R-S error control block buffer, may not be consecutive to each other in block 404. In FIG. 4, a ‘last’ packet 406 represents the packet last transmitted according to the format defined in the BOMA 400, and a ‘first’ packet 408 represents the packet first transmitted according to the format defined in the BOMB 402. However, if a transmitter transmits data after encoding the data such that data packets in R-S error control blocks before and after the change in BOM are consecutive, the receiver may lose the packets transmitted by the transmitter in a loss interval 404. That is, if the transmitter fills the loss interval 404 with data, the data in the loss interval 404 is not delivered to the receiver because the data is not actually transmitted.
In the conventional art, the change in the additional information causes a loss interval 404 of loss0˜loss12, in which data is not transmitted. As a result, performance is deteriorated because the receiver loses the data in this interval and cannot perform outer decoding even on the remaining data area of FIG. 4.
Accordingly, there is a need for an improved method and apparatus that prevents loss of information data transmitted and improves performance in a mobile communication system.